Expert: Arshad Khan - 12/2/2008

Question
QUESTION: Dear sir,
Thank U so much for your previous reply. I have few more questions to ask, i will really appreciate, if you could help me with your expertise.

1. while modelling mat foundaion in staad, we specify spring supports, at joints using subgrade modulus reaction, and perform analysis. The staad output gives the support reactions at the joints (i.e., force Y), after analysis.The question is what is the purpose of this reaction, how can i use it to evaluate the results obtained.

2. whats the difference between a portal frame model and normal space frame model ?

3. In a space frame model, is it good to assign pinned support or fixed support, please help!



ANSWER: Hi Ahmed,

1. From the support reactions, you can determine whether the soil bearing capacity has/has not been exeeded (divide the reaction by the tributary area- for example if you have a 2mx2m element, then divide the reaction by 4 to give pressure in kN/m2- actually STAAD has a feature to display the pressure contours).

2. No difference- portal is 2-D and SPACE is 3-D; if you model a portal frame by both ways (so long as there are only loads in 2-D), then the results will be identical.

3. This depends on how your foundation conditions are. If the columns are all on a stiff matt, then fixed is appropriate. If they are on small pad footings, with a chance of rotation/differential settlement, then it is better to consider supports as pinned or partially fixed.

---------- FOLLOW-UP ----------

QUESTION: Dear sir,
  I checked the base pressure contour, it comes around 6 to 7 kg/cm2,which exceeds the sbc value of 3kg/cm2,please advice


Answer
Hi Ahmed,

If the 3-7 value is very localised (small area), then you can ignore it. Also make sure you are comparing with service loads, not ultimate.

If there is still a problem, then you can lower the foundation so that the weight of the additional excavated material can be taken as extra bearing capacity- e.g. 2 m additional excavation can give you a net additional 30-36 kN/m2 bearing capacity.